Current demands for high density and performance associated with ultra large scale integration require submicron features, increased transistor and circuit speeds, and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring, including frequent and detailed inspections of the devices while they are still in the form of semiconductor wafers.
One important process requiring careful inspection is photolithography, wherein masks are used to transfer circuitry patterns to semiconductor wafers. Typically, a series of such masks are employed in a preset sequence. Each photolithographic mask includes an intricate set of geometric patterns corresponding to the circuit components to be integrated onto the wafer. Each mask in the series is used to transfer its corresponding pattern onto a photosensitive layer (i.e., a photoresist layer), which has been previously coated on a layer, such as a polysilicon or metal layer formed on the silicon wafer. The transfer of the mask pattern onto the photoresist layer is conventionally performed by an optical exposure tool such as a scanner or a stepper, which directs light or other radiation through the mask to expose the photoresist. The photoresist is thereafter developed to form a photoresist mask, and the underlying polysilicon or metal layer is selectively etched in accordance with the mask to form features such as lines or gates.
Fabrication of the mask follows a set of predetermined design rules set by processing and design limitations. These design rules define the space tolerance between devices and interconnecting lines and the width of the lines themselves, to ensure that the devices or lines do not overlap or interact with one another in undesirable ways. The design rule limitation is referred to as the critical dimension (“CD”, defined as the critical width of a line or the critical smallest space between two lines. The CD for most ultra large scale integration applications is on the order of a fraction of a micron.
As design rules shrink and process windows (i.e., the margins for error in processing) become smaller, inspection and measurement of surface features' CD, as well as their cross-sectional shape (“profile”) are becoming increasingly important. Deviations of a feature's CD and profile from design dimensions may adversely affect the performance of the finished semiconductor device.
One problem currently experienced in the art is that of systematic variation of CD in the reticles used to pattern the aforementioned photoresist mask, and resulting device feature. As those skilled in the art are well aware, any systematic variation of CD, normally less than about 40 nm in range, in the reticles will ultimately be projected to the final semiconductor features through the photoresist mask. Accordingly, reticle imperfections, such as systematic variation of CD in the reticle, can be costly to the manufacturing process.
It is believed that the systematic variation of CD in the reticles is caused by the inability of the device patterning the reticle (e.g., a reticle writing tool, such as vector-scan, shaped e-beam tools, JBX series of Jeol, EBM series of Toshiba, and HL series of Hitachi) to accurately calibrate itself after a required shift in the stage holding the reticle during patterning. Generally, the device patterning the reticle can only pattern a portion of the reticle before the stage must shift to continue patterning. During this shift, however, if the device is not re-calibrated accurately, a systematic shift in CD is created for the portion of the reticle formed after the shift. If not corrected, at least two different CD measurements may exist for the same feature. Unfortunately, by nature of the photolithographic process, these CD variations are transferred to the wafer. Even more unfortunate is the fact that no current reticle or mask inspection tools are capable of detecting such small CD errors in the reticle or mask.
Accordingly, what is needed in the art is a simple, cost-effective method or device that allows these systematic variation of CD in the reticles to be detected or corrected.